1. Field of the Invention
The present invention relates to a sheet processing apparatus that is connected to an image forming apparatus that forms images on the sheets and processes sheets discharged from the image forming apparatus, and an image forming system including the sheet processing apparatus and the image forming apparatus.
2. Description of the Related Art
In some cases, a sheet processing apparatus is connected to the sheet discharge side of an image forming apparatus, such as a copying machine, a printer, a facsimile machine, or a multi-function machine including these. Some sheet processing apparatuses of this type are configured to be able to stack and align sheets discharged from the image forming apparatus on a processing tray and then perform binding processing and the like processing on the sheets.
By the way, there has conventionally proposed an image forming apparatus provided e.g. with a correction device (hereinafter referred to as the lateral registration correcting device) configured to detect the side edge of a sheet in the transverse direction orthogonal to a sheet conveying direction and correct the transverse position of the sheet by transversely moving the sheet (see Japanese Patent Laid-Open Publication No. 2004-51256).
According to this proposal, it is possible to correct the transverse position of a sheet to thereby to cause the lateral registration position of the sheet to coincide with an image writing position. Further, since it is possible to detect a side edge of a sheet in the transverse direction and move the sheet during sheet conveyance, sheet position correction can be performed without reducing the productivity of the image forming apparatus. Furthermore, since sheets are subjected to lateral registration correction, each of the sheets can be discharged from the image forming apparatus into a sheet processing apparatus with the position of each side edge thereof in the transverse direction aligned.
In recent years, some systems, typically a high-speed digital copying system, have been configured to have various post-processing apparatuses, such as a casing-in bookbinding apparatus and a large capacity stacker, connected between an image forming apparatus and a sheet processing apparatus.
Therefore, even when a sheet is discharged from the image forming apparatus with the position of the side edge thereof in the transverse direction aligned, a positional shift of the sheet in the transverse direction can occur during conveyance of the sheet in the various post-processing apparatuses.
To cope with this problem, conventionally, sheets are aligned on a processing tray where the sheets are temporarily stacked before being processed, in a sheet processing apparatus. This means that it is required to align sheets on the processing tray even after execution of lateral registration correction in the image forming apparatus.
However, when the amount of a lateral registration shift of a sheet conveyed into the sheet processing apparatus exceeds a tolerable shift amount, the sheet can collide with an alignment plate of the processing tray to be damaged or cause paper jam. A high-speed digital copying system particularly necessitates system stability, and hence it is required to avoid occurrence of paper jam as thoroughly as possible.